According to a known technique the movement of a stream of material can be accelerated with the aid of centrifugal force. With this technique the material is fed onto the central part (the circular feed surface of a receiving and distributing member) of a rapidly rotating rotor and is then picked up by one or more accelerator members which are carried by the rotor with the aid of a support member and are provided with an acceleration surface that extends from the feed surface in the peripheral direction of the rotor. The material is accelerated along the acceleration surface, under the influence of centrifugal force, and, when it leaves the accelerator member, is propelled outwards at high velocity. Viewed from a stationary standpoint, after it leaves the accelerator member, the material moves at virtually constant velocity along a virtually straight stream that is directed forwards. Viewed from a standpoint moving with the accelerator member, after it leaves the accelerator member, the material moves in a spiral stream that is directed backwards, viewed in the direction of rotation. During this movement the relative velocity increases along the spiral path as the material moves further away from the axis of rotation.
The accelerated material can now be collected by a stationary impact member that is arranged in the straight stream that the material describes, with the aim of causing the material to break during the collision. The material strikes the stationary impact member at the velocity that it has when it leaves the rotor. The stationary impact member can, for example, be formed by an armoured ring that is arranged centrically around the rotor. The comminution process takes place during this single impact, the equipment being referred to as a single impact crusher. Such a device is disclosed in U.S. Pat. No. 5,248,101 (Rose). With this device the actual acceleration on the rotor takes place with the aid of accelerator members in the form of guide members which are arranged around the central part of the rotor. The guide members are provided with a guide surface that extends from the outer edge of the feed surface (central part) in the direction of the periphery of the rotor, usually in a radial direction in the case of single impact crushers. The known guide members are exposed to intense guide wear. Guide members are disclosed in, inter alia, U.S. Pat. No. 6,149,086 (Young), which describes a guide member that is secured with a heavy bolt, U.S. Pat. No. 6,179,234 (Marshall), which describes a specific mounting construction where the accelerator member is firmly anchored in the support member with the aid of centrifugal force, U.S. Pat. No. 5,921,484 (Smith), which describes a guide member that is provided along the guide surface with a cavity in which own material deposits, and WO 02/09878 A1 (Poncen), which describes a guide member that is provided along the guide surface with chambers that can be filled with hard metal.
U.S. Pat. No. 3,767,127 (Wood) discloses an accelerator member—which is of particular importance with regard to the accelerator member according to the invention—which is of symmetrical V-shaped construction and is provided with two acceleration surfaces, which V-shaped accelerator member has the point directed towards the axis of rotation and bears on a V-shaped support member, against which it anchors firmly under the influence of centrifugal force. A symmetrical accelerator member of this type has the advantage that the rotor is operational in both directions, as a result of which the tool life is doubled and the wear material is consumed more effectively, whilst as a result of the simple mounting the parts are very easy to replace and do not have to be specially secured.
Instead of allowing the material to impinge directly on a stationary impact member, it is also possible first to allow the material to impinge on a co-rotating impact member associated with the accelerator member, which co-rotating impact member is carried by the rotor and is arranged transversely in the spiral stream which the material describes, with the aim of allowing the material to collide once before the material strikes the stationary impact member. The material now impinges on the co-rotating impact member at the velocity which the material develops along the spiral path, the material being simultaneously loaded and accelerated during the impact, with which velocity the material is then loaded for a second time when it strikes the stationary impact member. With this arrangement there is said to be a direct multiple impact crusher, which has a much higher comminution intensity than a single impact crusher. A direct multiple impact crusher is disclosed in PCT/NL97/00565, which was drawn up in the name of the Applicant. The direct multiple impact rotor can also be of symmetrical construction, which makes it possible to allow the rotor to operate in both directions. A device of this type is disclosed in PCT/NL00/00668, which was drawn up in the name of the Applicant. It is also possible to allow the multiple impact crusher (and also the single impact crusher) to rotate about a horizontal axis instead of about a vertical axis. Such a device is disclosed in PCT/NL00/00317, which was drawn up in the name of the Applicant.
High forces are exerted on the accelerator members (and the support members) mainly by centrifugal force in the case of guide members and by a combination of (1) centrifugal force and (2) rapidly repeating impulse loading in the case of impact members. The centrifugal force increases progressively with (1) the rotational velocity and (2) the weight (mass) of the impact member, in which context a centrifugal force in excess of 100 kN can be considered under practical conditions. The impulse (impact) loading increases progressively with (1) the diameter (mass) and (2) the hardness (elasticity) of the impinging material, in which context grains with a weight of 1 to 2 kg which impinge repeatedly at a velocity of 50 to 100 m/sec can be considered under practical conditions.
Because the material from which the accelerator members are made must have a high resistance to wear, this material must be as hard as possible (Rc>55/60). Such a material is brittle and consequently not well able to withstand the tensile forces which are generated by the centrifugal loading and the impulse loading. Consequently, fracture can occur in the accelerator members, as a result of which part of the accelerator member, or the entire accelerator member, is propelled outwards at high velocity, which gives rise to a substantial imbalance. This can cause severe damage. Moreover, wear on the guide members is concentrated:                In the case of guide members a channel in which wear is concentrated forms fairly rapidly along the guide surface, as a result of which a deep channel forms fairly rapidly. This weakens the guide member, which can break as a result.        In the case of co-rotating impact members the movement (direction of movement) of the stream of material between the accelerator member and the co-rotating impact member is invariant (with respect to the rotational velocity) and is essentially deterministic. As a result, the material impinges on the co-rotating impact member in a highly concentrated manner. As a result a deep cavity can form fairly rapidly in the impact surface. The impact member is consequently severely weakened, as a result of which it can break.        
In order nevertheless to achieve a reasonable tool life, the known accelerator members must therefore be of extra heavyweight construction, so that no pieces start to break away when channels and cavities form. As a result of this additional weight, the mounting construction (and the support member) must also be made extra heavy, which makes the wear parts even heavier, and special provisions have to be made in order to fix the heavy accelerator member well to the support member. As a result of the low tensile strength of the hard, and consequently brittle, wear material, the accelerator members must for this be provided with extra heavy hooks and large projections and the mounting must be secured, for which bolts are often needed. All of this makes the replacement of the wear parts complicated and time-consuming, whilst the tool life, certainly in the case of abrasive material, remains restricted. An additional aspect that is certainly equally important is that a large amount of wear material remains; this is at least the additional portion that is needed to ensure that the accelerator member does not break and the additional structural material for the mounting. Frequently only 25% of the wear material is actually consumed.
As has been stated V-shaped accelerator members make a simple mounting possible. The problem, however, is that the stresses concentrate in the V-shaped pointed part. As a result fracture easily takes place at the location of the V-shaped point in the known V-shaped accelerator member, as a result of which the accelerator member breaks into two parts which are then propelled outwards. U.S. Pat. No. 3,652,023 (Wood) discloses a V-shaped accelerator member that is constructed as a triangle closed all round, which is provided with an opening in the middle, with the aid of which the accelerator member is mounted. An accelerator member of this type is stronger than an open V-shaped accelerator member, but the configuration demands a large amount of additional wear material that cannot be utilised. This type of V-shaped accelerator member is consequently not really effective. It is clear that a V-shaped accelerator member has major advantages. Nevertheless, despite numerous attempts to achieve this, effective utilisation of the V-shape has never been really successful because of the brittleness of the wear material.
In the case of the known guide members which are provided along the guide surface with one or more cavities in which own material deposits, a weak construction can be produced under the effect of wear, as a result of which fracture can occur. The same applies in the case of guide members where such cavities are filled with hard metal.
U.S. Pat. No. 3,346,203 (Danyluke) describes an autogenous rotor which is provided with an impeller vane with two arms which form pockets which fill with own material and act as an accelerating surface. To obtain a reasonable standtime the pockets are equipped with a notch which acts as a tip-end and channels the material from the autogenous accelerator surface to the next pocket and from there along a second tip-end (notch) out of the rotor. The notch consists out of a steel extension which is along one surface equipped with a lines of highly abrasive material to protect the notch, creating a sandwichconstruction. From the notch two sides, one of the liner and one of the sandwich extension/liner, act as sliding surface because the material moves around a corner of the notch (tip-end). Many other similar type tip-ends have been disclosed in the patent literature.
U.S. Pat. No. 6,033,791 (Smith et al) describes a wear resistant high impact iron alloy accelerating member, for accelerating particle material by sliding, that along the sliding surface is provided with an insert that is filled with a layer of carbide granules which are encapsulated in a matrix of white iron (the same white iron that is used for the white iron alloy member that is provided with the insert) to form a particle reinforced (large particle strengthened) composite accelerator member. The accelerator member and the layer are casted together; therefore the mold is (has to be) provided with a molding insert. The resulting casting is then heat treated (precipitation hardening) to obtain the required strength and hardness of the white iron. The molding insert must be capable of remaining in the cast member without significantly effecting its strength, impact resistance or wear resistance; that is, during casting and heat treatment. The result is a cast white iron alloy accelerator member having a high wear resistant region of matrix of particulate carbide contained in a selection location; here along the accelerating surface. This way the accelerator member exhibits and improved resistance to wear. To keep the layer in place when loaded by centrifugal forces and sliding and impact forces, the layer has to have a tensile strength that is at least as high as that of the white iron block that supports the layer. Therefore, it is for the known accelerator block most preferred to use tungsten carbide granules having a relatively high 12 to 18 weight % of cobalt content to achieve the required tensile strength (tensile strength of tungsten carbide increases progressively with the amount of cobalt added but the cobalt reduces the wear resistance; therefore, normally cobalt content is 5–10 weight %). Another problem with the known accelerator member is that the fixing member is made of (is part of) hardened white iron member (block) which is very hard but also very brittle and can therefore break off under influence of the high centrifugal forces and sliding and impact forces. To avoid this the velocity has to be limited; actually, this also applies for many other known accelerator members, which are provided with a similar fixing arrangement.